A Ni-based alloy is used in various fields because it has a high strength, high toughness, high corrosion resistance, and high heat resistance. One field using the Ni-based alloy is a natural gas fired power station which generates electric power by rotating a turbine by using a high-temperature gas generated by burning fuel or the like. The Ni-based alloy is used as a gas turbine member for use in this power station. Since the gas turbine member is placed in an environment to be exposed to a high-temperature high-pressure gas, constituent materials (e.g., a burner, stationary blade, moving blade, and transition piece) of the member are required to have a high heat resistance (e.g., a high-temperature strength and oxidation resistance) and a high toughness at a high temperature.
As a material having these characteristics, an age hardened Ni-based alloy defined by NCF750/751 of the JIS standard (JIS G 4902), containing, by mass, Cr: 14% to 17%, Fe: 5% to 9%, Ti: 2.25% to 2.75%, Al: 0.4% to 1%, and Nb+Ta: 0.7% to 1.2%, and also containing Ni and inevitable impurities as the balance is known. This Ni-based alloy is adopted as a gas turbine member having a high high-temperature oxidation resistance.
In contrast, as a material having a strength and high-temperature corrosion resistance higher than those of the above-mentioned NCF751, Patent Document 1 has proposed an Ni-based alloy for exhaust valves of an automobile engine and marine engine, which is an Ni-based alloy as an exhaust valve material or the like, contains, by mass, one or two elements selected from C: 0.01% to 0.20%, Si: 2% or less, Mn: 2% or less, Cr: 15% to 25%, Mo+1/2 W: 0.5% to 3.0%, Nb+Ta: 0.3% to 3.0%, Ti: 1.5% to 3.5%, Al: 0.5% to 2.5%, Fe: 5% to 15%, Zr: 0.01% to 0.10%, B: 0.0010% to 0.02%, Ca: 0.001% to 0.03%, and Mg: 0.001% to 0.03%, contains, in at %, Al+Ti+Nb+Ta=6.0% to 7.0%, and also contains Ni as the balance.
The constituent materials of the gas turbine are required to have different characteristics in accordance with portions where they are used. Therefore, Patent Document 2 has proposed a method of using Ni-based alloys in accordance with portions where they are used, in a power generating gas turbine including a compressor, a burner, and three or more stages of turbine blades and turbine nozzles fixed to a turbine disk.
For example, Patent Document 2 has disclosed a method in which (1) a first-stage turbine blade is made of a single-crystal cast of an Ni-based alloy, and turbine blades and turbine nozzles of the second and subsequent stages are made of casts of the Ni-based alloy, (2) a first-stage turbine blade is made of a single-crystal cast of a Ni-based alloy, a first-stage turbine nozzle is made of a Ni-based alloy unidirectionally solidified cast having a heat-shielding coating layer, and turbine blades and turbine nozzles of the second and subsequent stages are made of casts of the Ni-based alloy, (3) a first-stage turbine blade and first-stage turbine nozzle are made of a single-crystal cast of a Ni-based alloy, and turbine blades and turbine nozzles of the second and subsequent stages are made of casts of the Ni-based alloy, (4) a first-stage turbine blade is made of a Ni-based alloy unidirectionally solidified cast having a heat-shielding coating layer, and turbine blades and turbine nozzles of the second and subsequent stages are made of casts of the Ni-based alloy, and (5) a first-stage turbine blade and first-stage turbine nozzle are made of a Ni-based alloy unidirectionally solidified cast having a heat-shielding coating layer, and turbine blades and turbine nozzles of the second and subsequent stages are made of casts of the Ni-based alloy.
As the Ni-based alloy cast for use in the turbine blades of the second and subsequent stages, Patent Document 2 takes, as an example, a Ni-based alloy containing, by mass, Cr: 12% to 16%, Mo: 0.5% to 2%, W: 2% to 5%, Al: 2.5% to 5%, Ti: 3% to 5%, Ta: 1.5% to 3%, Co: 8% to 10%, C: 0.05% to 0.15%, and B: 0.005% to 0.02%. As the Ni-based alloy cast for use in the turbine nozzles of the second and subsequent stages, Patent Document 2 takes, as an example, a Ni-based alloy containing, by mass, Cr: 21% to 24%, Co: 18% to 23%, C: 0.05% to 0.20%, W: 1% to 8%, Al: 1% to 2%, Ti: 2% to 3%, Ta: 0.5% to 1.5%, and B: 0.05% to 0.15%.
Furthermore, Patent Document 3 has proposed a method of improving the creep characteristics by performing a heat treatment at a temperature of 1,650° F. to 1,850° F. for 0.5 to 2.0 hours when thermally treating a gas turbine member containing, by mass, Cr: 14.5% to 17.0%, Co: 12.0% to 15.0%, Mo: 2.50% to 5.05%, W: 0.5% to 1.5%, Ti: 4.0% to 5.5%, Al: 2.0% to 2.4%, Zr: 0.02% to 0.12%, C: 0.005% to 0.040%, B: 0.003% to 0.020%, and Mg: 0.001% to 0.005%, and also containing Ni as the balance.